1. Field of the Invention
The present invention relates to an image forming apparatus that forms an image by using developer.
2. Description of the Related Art
Electrophotographic color image forming apparatuses form a multicolor image by layering toners each having a different color. Therefore, so-called color misregistration could occur when the layering positions of the toners having different colors are misaligned from their respective ideal positions. One approach to reduce the color misregistration and to improve the color registration (alignment) is to actually form a color pattern in each color on the intermediate transfer belt, measure the amount of color misregistration (misalignment) of each color, and then adjust the image write start timing with respect to each color according to the amount of color misregistration. The intermediate transfer belt is extended under tension between a plurality of rollers, and rotates in a predetermined running direction (sub-scanning direction). When the plurality of rollers are not in parallel or the outer diameter of any of the rollers is not uniform, the intermediate transfer belt diagonally moves or meanders, and causes another color misregistration problem. To solve this problem, Japanese Patent Laid-Open No. 2008-281833 proposes a technology of detecting the inclination of the intermediate transfer belt between adjacent photosensitive drums, and adjusting the write start positions on the photosensitive drums. Japanese Patent Laid-Open No. 2010-85422 discloses a technology of measuring and recording the inclination of the intermediate transfer belt at the time of detection of the amount of color misregistration by using color registration patterns, and adjusting the write start positions by comparing the recorded inclination with the inclination at the time of image formation.
According to Japanese Patent Laid-Open No. 2008-281833 and Japanese Patent Laid-Open No. 2010-85422, the influence of the diagonal movement and meandering of the intermediate transfer belt is reduced by changing the offset values used for adjusting the write start position according to the inclination of the intermediate transfer belt. In order to accurately determine an offset value used for adjusting a write start position, it is necessary to obtain the inclination of the transfer belt immediately before the transfer of the corresponding toner image. However, there are cases in which the calculation of the write start position cannot be completed in time with respect to the color that is used first for forming a toner image, from among the plurality of colors. For example, when toner images in yellow, magenta, cyan, and black are sequentially transferred to the intermediate transfer belt in this order, there is the possibility that the write start position of the yellow toner image cannot be accurately determined. In order to accurately determine the offset value used for adjusting the write start position, it is necessary to detect the inclination of the transfer belt immediately before the transfer of the toner image. However, in some cases, a long time gap occurs from the detection of the inclination of the intermediate transfer belt to the start of the writing of the toner image, because of the waiting time for the rasterization of image data, the waiting time for the preparatory operations by the paper discharge unit, etc. In such cases, a difference occurs between the calculated inclination and the actual inclination, and this difference degrades the accuracy of the color registration adjustment.